Seating structures may be configured with flexible backrest members, such as polypropylene sheets and woven elastomeric membranes. Typically, the flexible members are put in tension in various ways so as to provide the flexible member with a three-dimensional contour.
Although a flexible backrest can accommodate movements and shifts in the users positions some users become uncomfortable after sitting for long periods of time. This is particularly true if the person sitting in the chair is restricted in the postural positions that he or she can assume for reasons such as the work task that they are performing.
The spine is broken down into four general regions: cervical (neck), thoracic (upper back), lumbar (lower back) and sacral (tail bone). In a natural aligned spine, the thoracic spine has a kyphotic, or gentle convex curve when viewed from the side. A lordotic lumbar spine, when viewed from the side, has a slightly concave curve. The sacrum is the base of the spine. It is a large triangular fusion of five vertebrae wedged between the pelvic bones. The connection of these bones is called the sacroiliac joint. Back problems can occur where the lumbar spine connects to the sacrum. The lumbar vertebrae, and the surrounding muscles, tendons and ligaments are also a common source of discomfort for a great many people after sitting for prolonged periods.
Sitting up straight can be difficult for the users of many chairs. In particular, in many seating devices a void exists between the backrest of the chair and the sacrum of a user. As a result, the user of the chair may take a slouched position. By slouching, the user will be placed in a position of poor posture, lack of muscle control and discomfort. Slouching can lead to a number of immediate problems. For example, increased fatigue or fidgeting may result because of discomfort. In addition, undesirable physical effects such as increased pressure on the lumbar discs or the creation of muscle spasms may also result from slouching. Various long-term problems may also occur. For example, pain in the lower back muscles or discomfort between the shoulder blades may result. Also, the tightening of neck muscles and muscle soreness and headaches may result.